In crystalline silicon solar cells using a single crystalline silicon substrate or a polycrystalline silicon substrate, high efficiency and price reduction thereof are important tasks.
FIG. 1 is a graph showing the structure of a crystalline silicon solar cell which is now mainly used. The mainly used crystalline silicon solar cell is provided with a crystalline silicon substrate 1, a diffusion layer 2, a surface antireflection coating 3, BSF (back surface field) layer 4, a first electrode 5 (an electrode on the light-receiving surface) and a second electrode 6 (an electrode on the back surface).
The electrode on the light-receiving surface (the first electrode 5 in FIG. 1) and the electrode on the back surface (the second electrode 6 in FIG. 1) are formed by coating with silver (Ag) paste and aluminium (Al) paste respectively, and calcining the resultant.
However, the mainly used crystalline silicon solar cells have the problems in that the substrate warps after calcination due to the difference in coefficients of thermal expansion between silicon and aluminum, especially on the back surface, carrier recombination is large and reflectance is small. These problems are obstacles to high efficiency of solar cells. Further, in cases where the thinning of solar cells are to be attempted, these problems are more noticeable obstacles to the high efficiency of solar cells.
As a means for solving such problems, back side contact solar cell structures in which an electrode is not formed on the entire back surface of the crystalline silicon solar cell with Al paste, but formed on parts of the back surface, and other parts of the back surface are covered with backside passivation layer (also referred to as back reflection layer) such as a silicon oxide film or a silicon nitride film (SiN film) are proposed (Non-patent Literatures 1 and 2). However, the means proposed in the Non-patent Literatures 1 and 2 is not preferred in view of the cost, since a contact is provided by holing the film with photolithography and etching after forming a silicon oxide film or a SiN film on the back surface. Moreover, Patent Literature 1 discloses a method using a dicing saw and a method using a laser for providing a contact. However, the method of making contact holes after coating the entire surface with the film has the problem in that the production process is complicated.
The heat-resistant resins represented by polyimide resins have been widely used as surface protective films or interlayer insulation films of semiconductor devices in the field of electronics since they have superior heat resistance and mechanical characteristics. On the other hand, Patent Literatures 3 and 4 describe in detail a method of producing a first passivation layer and second passivation layer formed on the surface of a solar cell. Solar cells are known devices for converting solar radiation to electrical energy. The solar cells can be produced on a semiconductor wafer using semiconductor processing technique.
Further, the improvement of reflectance on the back surface is attained by increasing reflection from passivation layers and electrodes formed with aluminium or silver. For example, in Patent Literature 1, the reflection on the back surface is increased by restricting the thickness of SiN film (“silicon nitride film” in Patent Literature 1) formed on the back surface. However, the SiN film used on the back surface is produced mainly by chemical vapor deposition method (CVD method), so that it has a problem in that the production cost is high.